Pelvic Incidence (PI)

How is pelvic incidence measured?

Use a standing lateral radiograph that includes both femoral heads.

1. Mark the midpoint of the S1 superior endplate.
2. Draw a line perpendicular to the endplate at that point.
3. Draw a line from the endplate midpoint to the center of the femoral head axis. If the two femoral heads do not overlap, use the midpoint between their centers.
4. PI is the angle between these two lines.

PI was described by Duval-Beaupère and popularized by Legaye and colleagues in 1998 as a fundamental parameter regulating sagittal spinal curves.

What is the normal range?

In the multi-ethnic MEANS study (468 asymptomatic adults), mean values were:

• PI: 52.0° ± 10.7°
• PT: 12.5° ± 7.3°
• SS: 39.5° ± 8.2°
• L1–S1 lordosis: 57.4° ± 11.3°

Across populations, PI ranges from roughly 35° to 85°, and it tends to be higher in women. There is no single "normal" PI. What matters is whether lordosis matches it.

PI vs pelvic tilt vs sacral slope

PI is morphologic: it doesn't change with posture. PT and SS are positional: they shift as the pelvis rotates around the femoral heads, but they always sum to PI. When lordosis is lost, the pelvis retroverts, so PT rises and SS falls. A high PT is therefore a marker of compensation.

Does PI change with age?

PI is generally considered constant once growth is complete. However, it may increase modestly in older adults, possibly from sacroiliac stress, particularly beyond age 75.

What is PI–LL mismatch?

PI–LL mismatch is the difference between PI and lumbar lordosis. A value under about 10° is the traditional target. In the SRS–Schwab classification, the sagittal modifiers are:

Thresholds associated with severe disability (ODI >40) are PT ≥22°, SVA ≥47 mm, and PI–LL ≥11°.

Should alignment targets account for age?

Yes. Lafage and colleagues (2016) showed that the alignment associated with age-normative quality of life loosens with age. The ideal PI–LL is about −10° under age 35 and about +14° over age 74. Undercorrection relative to age-adjusted targets worsens outcomes. Overcorrection adds no clinical benefit and is associated with proximal junctional failure.

Where should lordosis be restored?

Dr. Diebo's 2024 JBJS review addresses how lordosis is distributed. Caudal (L4–S1) lordosis is relatively constant across patients. As PI increases, a larger share of lordosis comes from the cranial levels (L1–L4). Fusing without attention to both the magnitude and the location of lordosis risks iatrogenic deformity.

Common pitfalls

• Femoral heads that do not overlap on the image, so the midpoint is not used
• Lumbosacral transitional vertebrae, which change which level counts as S1
• Hip arthritis or flexion contracture, which can limit pelvic retroversion. This interaction is addressed in Dr. Diebo's 2019 hip–spine classification.

Learn more about the PI Rod™ System.

← For clinicians